[time-nuts] Anybody want a Thunderbolt power supply?

Bob Camp kb8tq at n1k.org
Sat Oct 22 09:06:53 EDT 2016


Hi

The +12 goes straight to the OCXO. All OCXO’s have a voltage sensitivity. 
That sensitivity is much higher than voltage sensitivity is much higher than 
what you see on the other two supply pins.

Bob

> On Oct 21, 2016, at 11:06 PM, Scott Stobbe <scott.j.stobbe at gmail.com> wrote:
> 
> Interesting, I would have thought that the +12V input would be extremely
> well regulated since its shared with the oven heater, I*R drops are going
> to show up every where, if your looking for uV levels of stability.
> Just a connector has milliohms of contact resistance, let alone routing and
> cables...
> 
> On Friday, 21 October 2016, Bob Camp <kb8tq at n1k.org> wrote:
> 
>> Hi
>> 
>>> On Oct 21, 2016, at 7:44 PM, Scott Stobbe <scott.j.stobbe at gmail.com
>> <javascript:;>> wrote:
>>> 
>>> A little more data on the 7912.
>>> 
>>> The first plot shows the tempCo of the 7912 measured with ambient
>>> temperature swings "7912_TempCo.png". Which is -150 ppm/degC.
>>> 
>>> The second plot is off a 7912 logged for an hour or so, "7912_1PLC.png",
>>> nothing too interesting here. However the environmental temperature swing
>>> of about 1 degC/hour is pretty conservative for a DUT sitting in free
>> air.
>>> 
>>> Finally, an allan devation plot looking at the normalized stability of a
>>> 7912 regulator "7912_AllanDeviation.png". Interestingly here, is, how
>> quick
>>> a 15 mK/min temperature swing shoots above the 1/f floor, it's a matter
>> of
>>> seconds.
>>> 
>>> Now if your PSRR is 1 ppb/V or better, then all of this is comfortably
>>> below the intrinsic noise of a thunderbolt.
>> 
>> The main point is that the internal tempco of the TBolt it’s self is much
>> larger than
>> the issues surrounding the power supply pins. The +12 is the only one that
>> is
>> sensitive enough to voltage (change in frequency vs voltage) to contribute
>> to any
>> significant way to the overall stability.
>> 
>> Bob
>> 
>>> 
>>> On Fri, Oct 21, 2016 at 12:20 AM, Scott Stobbe <scott.j.stobbe at gmail.com
>> <javascript:;>>
>>> wrote:
>>> 
>>>> Nick had mention that the -12V rail on the thunderbolt has the poorest
>>>> PSRR with respect to frequency output, so I first took a look at the
>>>> venerable 7912.
>>>> 
>>>> The first data-set was taken with a -13.5 VDC input. Attached is the 0.1
>>>> Hz to 10 Hz noise of an essentially quiescently loaded 7912, only a 10k
>>>> resistor was added as load for preliminary evaluation. With a 60 dB
>> preamp
>>>> the scale of the scope plot is 20 uV/div. The 0.1Hz to 10Hz band noise
>> is
>>>> 15 uVrms, which is about 1.3 ppm rms of the DC mean.
>>>> 
>>>> In allan deviation terms, a quiescently loaded 7912 has a spot noise of
>> 7
>>>> uV/rtHz at 1 Hz (on the 1/f slope), normalized that's 580 ppb/rtHz.
>>>> Equivalently speaking, the flicker noise floor of an allan deviation
>> plot
>>>> would be sqrt(2*ln(2)) that figure to be 6.8E-7.
>>>> 
>>>> Assuming a thunderbolt should be achieving 1/f floor of around 1E-12, it
>>>> would need a PSRR of at least 1 ppm/V. I'm sure someone has gone to the
>>>> trouble of actually measuring it.
>>>> 
>>>> So from a 0.1 Hz to 10 Hz noise standpoint, the 7912 isn't terrible
>>>> with 1.3 ppm rms noise, considering an LM399 is about 0.1 ppm rms, only
>> one
>>>> order of magnitude off.
>>>> 
>>>> The bad side of a 7912 is in long-term stability and tempCo, the sample
>> I
>>>> tested had at least a 150 ppm/degC tempCo, which is going to put a
>> serious
>>>> lump/bump in the 10s tau to gps crossover point on an allan deviation
>> plot.
>>>> 
>>>> 
>>>> 
>>>> On Tue, Oct 18, 2016 at 3:05 PM, Scott Stobbe <scott.j.stobbe at gmail.com
>> <javascript:;>>
>>>> wrote:
>>>> 
>>>>> I'm sure I have some 7805s lying around, maybe a 7812/7912. I'm
>>>>> interested to see the 1/f noise of a classic regulator, what load
>> current
>>>>> do you expect? I can bias a 7805 for the same load and measure the 0.1
>> to
>>>>> 10 Hz noise.
>>>>> 
>>>>> Also if you have a digital scope without a very good builtin FFT,
>> octave
>>>>> would be one solution.
>>>>> 
>>>>> On Tue, Oct 18, 2016 at 10:46 AM, Nick Sayer via time-nuts <
>>>>> time-nuts at febo.com <javascript:;>> wrote:
>>>>> 
>>>>>> Just an update. I’ve built the second prototype board (I skipped over
>>>>>> the first design), and it’s powering my tbolt right now.
>>>>>> 
>>>>>> The design calls for 15v in (though it would also work with 13.8v).
>> The
>>>>>> +12 output comes from a D2PAK 7812. For +5, there is an AP1509 buck
>>>>>> converter to make around 6.5 volts, then a DPAK 7805. For -12, there
>> is an
>>>>>> MC34063 configured as an inverter to make around -13.75 volts and
>> then a
>>>>>> DPAK 7912.
>>>>>> 
>>>>>> Steady-state, the system appears to be working just fine. The AP1509’s
>>>>>> inductor and the D2PAK 7812 are just warm to the touch.
>>>>>> 
>>>>>> I checked for noise and ripple on the outputs and it’s somewhere
>> around
>>>>>> ±2 mV or so generally. From what I can see on the scope, there’s no
>> ripple
>>>>>> - it’s all high frequency noise. I am not absolutely certain that the
>> noise
>>>>>> measurement represents real noise or the limits of my measuring
>> ability.
>>>>>> I’m just using the scope probes the scope came with, and 2 mV/div is
>> its
>>>>>> lowest range.
>>>>>> 
>>>>>> I haven’t compared the noise with the ex laptop supply that I was
>> using
>>>>>> before, but I’d have to believe it’s cleaner. I don’t really have a
>> way to
>>>>>> check the oscillator’s before and after ADEV. My only other reference
>> is an
>>>>>> FE5680A, and I think the thunderbolt’s going to be far better at
>> lower tau
>>>>>> (where this all matters).
>>>>>> 
>>>>>> I know also that ±2 mV is still one and perhaps two orders of
>> magnitude
>>>>>> higher than some have called for. But before I attempt to reduce the
>> noise
>>>>>> further, I’d like to know that there are real gains to be had. Would
>>>>>> someone with a Thunderbolt and better output noise measuring
>> wherewithal be
>>>>>> willing to take a prototype and compare it with something that does
>> have µV
>>>>>> levels of noise and ripple so I can get an idea of what there is to
>> gain?
>>>>>> If you like, you can make such comparisons public - no secrets here.
>>>>>> 
>>>>>>> On Aug 30, 2016, at 10:37 PM, Nick Sayer <nsayer at kfu.com
>> <javascript:;>> wrote:
>>>>>>> 
>>>>>>> 
>>>>>>>> On Aug 30, 2016, at 8:48 PM, Cube Central <cubecentral at gmail.com
>> <javascript:;>>
>>>>>> wrote:
>>>>>>>> 
>>>>>>>> I would be interested, I think.  Planning ahead for if the one I
>> have
>>>>>> for my Thunderbolt fails, I guess.  Are there different models or
>> would a
>>>>>> photo of the input ports on mine be useful?
>>>>>>> 
>>>>>>> Actually, what I had in mind is to just put a SIP4 header on the
>> board
>>>>>> for the output and people could wire the “last mile” themselves. The
>> input
>>>>>> is a 2.1mm barrel connector. You use whatever 15W 12VDC wall wart is
>> handy
>>>>>> and plug it right in.
>>>>>>> 
>>>>>>> What it really amounts to is that you get +12 volts directly from the
>>>>>> input, then there’s a buck converter to drop the +12 down to +5 and an
>>>>>> inverter to generate -12 from the +12. Those 3 voltages, plus a
>> ground go
>>>>>> to the SIP4.
>>>>>>> 
>>>>>>> So it’s just two switching power supplies to turn a +12 volt only
>>>>>> supply into the three-way that the Thunderbolt wants.
>>>>>>> 
>>>>>>> It’d be good for around 1500 mA @ 5V and around 50 mA @ -12 (the +12
>>>>>> spec is whatever is left from the source supply’s power spec) - more
>> than
>>>>>> enough for a Thunderbolt. Probably enough for a hard disk or a
>> smallish PC.
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>>>>>> 
>>>>> 
>>>>> 
>>>> 
>>> <7912_1PLC.png><7912_AllanDeviation.png><7912_
>> TempCo.png>_______________________________________________
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